1. Field of the Invention
The invention relates to a controller for a brushless direct current motor, more particularly to a controller that does not require Hall components to detect rotor position for current control of a brushless direct current motor.
2. Description of the Related Art
A brushless direct current motor generally includes a rotor having north and south magnetic poles, and a stator. By conducting electricity through coils on the stator, magnetic repulsion forces can be generated to drive the rotor to rotate. In a conventional motor driving circuit, a position sensing mechanism, such as a Hall IC, is adopted to determine positions of the north and south magnetic poles of the rotor so that the direction of electric currents flowing through the stator coils can be varied to ensure continuous rotation of the rotor.
FIG. 1 illustrates a conventional controller for a brushless direct current motor, which employs a sensing coil (L2) instead of a Hall IC. The sensing coil (L2) is used to sense magnetic position of the rotor (not shown), and controls on/off states of transistors (Q1, Q2) accordingly.
With further reference to FIGS. 2 and 3, assuming that the rotor is disposed such that the sensing coil (L2) is able to sense a south magnetic pole (S) of the rotor, an electric current (as indicated by the solid arrow line in FIG. 1) is induced in the sensing coil (L2), which triggers the transistor (Q2) to conduct, thereby cutting-off the transistors (Q1). At this time, electric current is unable to flow through a drive coil (L1) on the stator, and magnetic repulsion forces for driving rotation of the rotor are not generated, thereby resulting in deceleration of instantaneous rotor speed. On the other hand, when the rotor is disposed such that the sensing coil (L2) is able to sense a north magnetic pole (N) of the rotor, the transistor (Q2) will be cut-off, whereas the transistors (Q1) will conduct. At this time, electric current (as indicated by the dotted arrow line in FIG. 1) is able to flow through the drive coil (L1) on the stator, and magnetic repulsion forces for driving rotation of the rotor are generated, thereby resulting in acceleration of the instantaneous rotor speed. By supplying electric current to the drive coil (L1) only when the sensing coil (L2) senses a specific magnetic pole (i.e., the north magnetic pole) of the rotor, the rotor could be controlled to rotate at a rated (average) speed.
However, the aforementioned conventional controller does not permit adjustments in the rated (average) speed. Moreover, as best shown in FIG. 3, in view of the large fluctuation in the instantaneous rotor speed, the rated (average) speed of the rotor is actually relatively low.